Tofu or soymilk is a traditional health food highly nutritious and made from soybeans. In recent years, tofu or soymilk that is a natural food has had received attention for health-conscious consumers. On the other hand, while soybean products have come under review as least favorite, traditional and healthy foods because of an unpleasant smell and an astringent taste peculiar to tofu or soymilk, soymilk suppressed in inherent unpleasant smell and astringent taste and having ever better quality has been expected.
The unpleasant smell peculiar to soybeans is represented by a grassy smell that is a rancid smell as being emitted from green grass and has a lot to do with the action (oxidation and degradation reactions) of lipoxygenase that is one of enzymes. The lipoxygenase reacts with unsaturated fatty acid contained in soybeans to produce lipid peroxide, and the lipid peroxide is further decomposed into medium-chain aldehyde and alcohols constituting an unpleasant smell (grassy smell). In order for the lipoxygenase to act, the presence of lipid constituting a substrate and oxygen is needed. According to academic document 1, lipoxygenase contains three kinds of isozymes L1, L2 and L3, of which L2 particularly affects the generation of a grassy smell, and the deactivation temperature of the isozyme is 60° C. at which the enzymatic activation is reduced by one half. In addition, β-glucosidase that is a glycolitic enzyme separates sugar of glycosides of saponin or isoflavonoid contained in soybeans to produce aglycone stronger in unpleasant taste and astringent taste. According to document 1, the deactivation temperature of β-glucosidase is 50° C. In addition, it has been known that the action of these enzymes is suppressed at 15° C. or less. That is to say, there have been known plenty of techniques for suppressing an unpleasant smell of soymilk or tofu, including a method of subjecting soybeans to pretreatment of high-temperature heating (Patent Document 4), an enzyme deactivation treating method that grinds down soybeans by friction in hot water (Patent Document 5), an alkaline-side adjustment treating method using a pH adjuster (Patent Documents 4, 9 and 10), a method of adding an enzyme-inhibiting agent (Patent Document 7), and a method of grinding down soybeans by friction in an anoxic or low-temperature environment (Patent Documents 3, 6 and 9). Though a method comprising adding hot water directly to soybeans to deactivate lipoxygenase has been reported, the method poses a problem of lowering the extractability of a soymilk solid content because proteins stored in the soybeans are immobilized. In addition, though it is effective that soybeans are heated to a predetermined temperature immediately after being ground down by friction, thereby deactivating the oxidizing and degrading enzymes (Patent Document 2), this method is of ill report because it entails failure to coagulate tofu even by the addition of a coagulant or mere production of tofu that is soft even when being coagulated or flavorless and bland.
As another method, there is a method of using in some locations soybeans lacking the gene for lipoxygenaze as raw materials. However, since the soybeans that lack the gene for lipoxygenaze are more expensive by 30 to 40% than ordinary soybeans, the method entails the problem resulting in an increase in cost to a great extent.
Under these circumstances, an improvement in an apparatus for producing soymilk, in which soybeans are ground down by friction in a low-oxygen atmosphere to obtain a raw soybean soup which is then immediately heated to a predetermined temperature, has been in progress. Disclosed in Patent Document 1 is a method for producing smell-less soymilk that comprises grinding down soaked soybeans by friction in an anoxic atmosphere to obtain a raw soybean soup, then transferring the raw soybean soup to an instantaneous continuous heating unit in a state of noncontact with air and continuously heating the raw soybean soup under conditions of a temperature of around 80° C. or more and a period of around one sec or more, with the instantaneous continuous heating unit filled with the raw soybean soup. Furthermore, in Patent Document 2, an apparatus for continuously grinding down food grains by friction and a method for producing soymilk comprising grinding down soybeans by friction in water to obtain a raw soybean soup, subsequently feeding the raw soybean soup from a connecting pipe connected to a friction-grinding portion to a transferring pipe with a transfer pump and heating the raw soybean soup to a predetermined temperature in a heating unit are disclosed.
Patent Document 1: JP-A SHO 52-154545
Patent Document 2: JP-A 2005-304474
Patent Document 3: JP-A SHO 52-154545
Patent Document 4: JP-A SHO 52-125667
Patent Document 5: JP-A SHO 53-047553
Patent Document 6: JP-A SHO 62-502864
Patent Document 7: JP-A SHO 51-029280
Patent Document 8: JP-A SHO 64-086851
Patent Document 9: JP-B SHO 56-050818
Patent Document 10: JP-A SHO 53-091161
Patent Document 11: JP-A HEI 05-184293
Patent Document 12: JP-A SHO 53-066465
Patent Document 13: JP-A 2006-015206
Non-Patent Document 1: Food Science and Technology Research, Vol. 36, No. 4, pp. 318-324 (1989)
Non-Patent Document 2: Food Science and Technology Research, Vol. 36, No. 8, pp. 658-663 (1989)
Non-Patent Document 3: Report of the Soy Protein Research Committee, Vol. 15, pp. 36-40 (1994)